FIG. 1 shows a part of a telecommunication network 10. The radio access network 10 comprises a plurality of radio base stations 11 (whereof only one is shown in the figure), each of which communicates with a plurality of UEs (user equipments) 12 located in the coverage area of the radio base station. The base station 11 further communicates with a core network 13. For example, where the network 10 is a standardized E-UTRAN (Evolved UMTS Terrestrial Radio Access Network), the core network 13 comprises an evolved packet core, itself comprising a mobility management entity (MME), a serving gateway and a PDN (packet data network) gateway.
The E-UTRAN currently supports bandwidths up to 20 MHz. However, one of the requirements of future releases of this standard such as LTE Advanced is the support of bandwidths larger than 20 MHz. A further important future requirement is to assure backward compatibility with previous releases. This also includes spectrum compatibility. That implies that a future-release carrier, wider than 20 MHz, appears as a number of carriers to a legacy UE. Each such carrier can be referred to as a component carrier. In particular for early deployments of future releases, it can be expected that there will be a smaller number of future-release UEs compared to many legacy UEs. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy UEs, i.e. that it is possible to implement carriers where legacy UEs can be scheduled in all parts of the wideband future-release carrier. The straightforward way to obtain this is by means of carrier aggregation. Carrier aggregation implies that a future-release UE can receive multiple component carriers, where the component carriers have, or at least have the possibility of having, the same structure as a carrier of previous releases.
Different examples of carrier aggregation are illustrated in FIGS. 2a to 2c. A contiguous intra-band carrier aggregation is illustrated in FIG. 2a where five component carriers 20, each of 20 MHz bandwidth, have been aggregated to form a bandwidth of 100 MHz. FIG. 2b illustrates a non-contiguous intra-band carrier aggregation where three component carriers 20, each of 20 MHz bandwidth, have been aggregated together to form an aggregated bandwidth of 60 MHz. Finally, FIG. 2c illustrates a inter-band carrier aggregation where two component carriers 20 in band x and band y respectively, each of 20 MHz bandwidth, have been aggregated together to form an aggregated bandwidth of 40 MHz.
The number of aggregated component carriers as well as the bandwidth of the individual component carrier may be different for uplink (UL) and downlink (DL). A symmetric configuration refers to the case where the number of component carriers in DL and UL is the same whereas an asymmetric configuration refers to the case that the number of component carriers is different. It should be noted that the number of component carriers configured in a network may be different from the number of component carriers seen by a UE. A UE may for example support more DL component carriers than UL component carriers, even though the network is configured with the same number of UL and DL component carriers.
The current E-UTRAN but also LTE Advanced uses DFTS-OFDM (Discrete Fourier Transform Spread—Orthogonal Frequency Division Multiplex) for modulation in the UL. DFTS-OFDM is a special kind of FDM (Frequency Division Multiplex) where different users are assigned different portions of the spectrum. Orthogonality among different users relies on the time-aligned arrival of the UL signals of the various users. In current E-UTRAN and LTE Advanced a cyclic prefix is used which relaxes the requirement on timing alignment slightly. Hence, it is sufficient if the signals are aligned within a fraction of the cyclic prefix. The normal cyclic prefix in LTE is around 5 μs; signals from different users should then be aligned within 1 μs or so.
UEs synchronize their DL timings to DL signals transmitted from an eNodeB, i.e. a radio base station of an E-UTRAN. Signals used for this purpose comprise synchronization signals and reference signals. After established downlink synchronization a UE can start to transmit signals in the UL from the UE to the eNodeB at a well defined offset relative to the DL timing. However, due to varying UE-eNodeB distances among UEs the synchronization signals arrive at different time instances at the UEs creating UE specific DL timings at each UE. This results in different transmission instances among the UEs. UEs close to the eNodeB receive the synchronization signal early and thus also start to transmit early; their respective UL signals require furthermore only a short propagation time to arrive at the eNodeB. UEs far away from the eNodeB start to transmit later and their UL signals require also more time to traverse the distance to the eNodeB resulting in later signal arrival times at the eNodeB. The time difference between arriving signals from two UEs is the difference in round trip time between these two UEs. Round trip time is defined as the time duration that is needed for a signal to traverse the distance eNodeB-UE and back (assuming zero processing delay at the UE) and is calculated as twice the distance eNodeB-UE divided by the speed of light.
In order to align the arrival times of UL signals from different UEs the eNodeB measures the arrival times of the different signals relative to a desired arrival time reference and informs the UEs by how much they have to advance/delay their UL transmission timings (either relative to their UE specific DL timings or to the current UL transmission timing). This process is called timing alignment procedure and the command used to notify the UE about the required correction is called timing advance command.
In case of a UE with completely unsynchronized UL timing the UE does not transmit a regular UL signal but a random access signal. This signal is specifically designed for unsynchronized UEs. After the eNodeB has determined the required correction of UL transmit timing it transmits a corresponding timing advance command to the UE which corrects its UL timing accordingly. After that the UE can now start to transmit regular UL signals which assume aligned arrival timings. The eNodeB continuously monitors UL signal arrival timings and sends timing advance commands to maintain a valid UL timing. If a UE is not active in the UL for a long time or looses UL synchronization for another reason a new random access needs to be performed to establish a valid UL timing again.
In case of UL carrier aggregation all UL signals within a component carrier and/or across contiguous component carriers need to be timing aligned in order to maintain orthogonality.